We are developing a prophylactic HIV-1 vaccine that aims to protect against all clades of HIV-1 through heterologous prime/boost regimens using adenovirus serotype 26 (Ad26) vectors expressing unique mosaic Gag, Pol, and Env antigens with soluble trimeric Env gp140 proteins to enhance humoral immunity. In December 2014, the program entered into its Phase 1/2a clinical phase assessing the safety and immunogenicity of Clade C gp140, trivalent Ad26.Mos.HIV vectors, and MVA.Mos.HIV vectors. Additional Phase 1/2a clinical studies started in 2016 and early 2017 are testing a tetravalent Ad26.Mos4.HIV and the addition of a Mosaic gp140 protein to the Clade C gp140 component. In an effort to explore simplification of the vaccine components, we propose to conduct additional ancillary clinical studies. Priority will be given to combinations evaluated in Project 1, including simplified vaccine composition. To secure availability of clinical trial materials, we propose GMP manufacturing activities that cover fill and finish, release assays, and characterization and stability testing of drug products. Finally, in order to support late stage development and characterization of selected vaccine candidates, we propose to perform bridging preclinical studies. In this Project, we hypothesize that our lead Ad26/Env vaccine will protect humans against multiple strains of HIV-1. Moreover, we hypothesize that simplified and shortened regimens will prove comparably immunogenic. To test these hypotheses we propose the following Specific Aims: Specific Aim 1. To progress with the clinical evaluation of the lead candidate vaccines from Phase 1/2a to Phase 2b and Phase 3 clinical trials while in parallel exploring shorter and/or simplified regimens Specific Aim 2. To manufacture, release and characterize clinical vaccine products and to support CMC development with preclinical bridging studies